1. Technical Field
The present invention relates to position regulation and is directed to an apparatus for regulating the position of a drive shaft for driving one unit of a printing press relative to the position of another drive shaft for driving another unit of the printing press during acceleration of the drive shafts.
2. Background Art
A printing press typically includes a number of printing units. In certain printing presses, each printing unit has a drive shaft which is drivingly connected with an output shaft of an electric motor for driving the drive shaft of that particular printing unit. A speed controller provides a speed control signal to control the rotational speed of the output shaft of the electric motor. The speed controller provides the speed control signal in response to a control output signal from a regulator. The control output signal from the regulator is generated in response to a signal indicative of the actual position of the drive shaft and another signal indicative of the actual position of another drive shaft of another printing unit, known as the master drive shaft.
The position of the drive shaft of each printing unit relative to the master drive shaft must be accurately controlled and maintained so as to maintain proper registration of the different printing units. Since the drive shaft of each printing unit is drivingly connected with the output shaft of its associated electric motor, the position of each output shaft relative to the master drive shaft must be accurately controlled and maintained so as to maintain proper registration of the different printing units. Proper registration of the different printing units is required to prevent errors such as printing registration errors, web tension errors, web-to-web registration errors, and/or signature cutoff errors.
Many types of regulators are available for regulating the position of a drive shaft relative to the position of a master drive shaft. One type of regulator for regulating the position of a drive shaft relative to the position of a master drive shaft is a phase-locked loop regulator in which the output signal of a closed-loop servomechanism locks onto the tracks a reference signal. Phase lock is accomplished by comparing the phases of the output signal and the reference signal. Any phase difference between the output signal and the reference signal is converted into an error correction signal. The error correction signal is used to modify the output signal of the closed-loop servomechanism so that the output signal tracks the reference signal.
Another type of regulator for regulating the position of a drive shaft relative to the position of a master drive shaft is a synchro regulator. In this type of regulator, a device known as a synchro is used to transform the angular position of the drive shaft into an electrical output signal indicative thereof. The position of the drive shaft relative to the position of the master drive shaft is regulated in response to the electrical output signal from the synchro regulator.
If a regulator such as the phase-locked loop regulator or the synchro regulator is used, an undesirable position error between the actual position of the drive shaft and the actual position of the master drive shaft results during acceleration of the printing press. This relative shaft position error during acceleration of the printing press is a steady-state error. This position error during acceleration of the printing press is undesirable because it may result in undesirable errors such as printing registration errors, web tension errors, web-to-web registration errors, and/or signature cutoff errors.
The value of the shaft position error during acceleration of the printing press could be reduced or even theoretically eliminated by using a regulator with open-loop compensation. Open-loop compensation is often referred to by other names such as "forcing", "speed reference", and "dp/dt feed-forward". Although a regulator with open-loop compensation may initially reduce the value of the shaft position error, such a regulator is subject to undesirable long-term component variation or hardware modification for maintenance.